1. Field of the Invention
The present invention relates to a sheet feeding apparatus configured to feed a sheet (document), an image reading apparatus including the sheet feeding apparatus, and an image forming apparatus including the image reading apparatus.
2. Description of the Related Art
Hitherto, there is known an image reading apparatus including an automatic document feed apparatus (ADF) enabling to skim image information, i.e., to read the image information, of a document passing on a platen glass by an optical system disposed under the platen glass. There is also known a so-called reverse reading type image reading apparatus configured to switch-back and to reversely convey a document whose image of a first face has been read by a discharge roller pair to pass on the platen glass again and to read an image of a second face on a back of the first face.
Here, if a length of a sheet whose image is to be read exceeds a predetermined range in the case of the reverse reading type image reading apparatus, there is a case where a front end of the switch-backed document arrives at a discharge roller pair before a rear end of the document passes through a nip of the discharge roller pair, thus becoming incapable of conveying the document. Therefore, it is necessary to release the nip of the discharge roller pair at predetermined timing to pass through the document in reading both-side images of a document whose length exceeds a predetermined length.
As a method for releasing the nip of the discharge roller pair, Japanese Patent Application Laid-open No. H11-263472 discloses a technology of connecting a solenoid to a driven discharge roller driven by a driving discharge roller and releasing the nip of the discharge roller pair by separating the driven discharge roller from the driving discharge roller by the solenoid. However, a relatively large solenoid is required to separate the driven discharge roller from the driving discharge roller directly by the solenoid by resisting against a nip pressure of the discharge roller pair. Accordingly, this poses a problem that the large solenoid as described above is required in addition to such a motor that rotates the discharge roller pair and that the apparatus is thus enlarged.
Meanwhile, Japanese Patent Application Laid-open No. 2005-335915 discloses a technology including a cam mechanism enabling to contact/separate a driven discharge roller to/from a driving discharge roller and separating the driven discharge roller from the driving discharge roller by transmitting power from the motor not only to the driving discharge roller but also to the cam mechanism through a gear train.
Here, the cam mechanism disclosed in Japanese Patent Application Laid-open No. 2005-335915 is provided with an electromagnetic clutch between the gear train and the cam mechanism to transmit the power from one motor to both of the driving discharge roller and the cam mechanism. This technology makes it possible to contact/separate the driven discharge roller with/from the driving discharge roller at any timing without repeating the contact/separation of the rollers during when the driving discharge roller is driven by disconnecting/connecting the transmission of the power from the motor to the cam mechanism by the electromagnetic clutch.
However, the configuration using such an electromagnetic clutch has problems that a structure of the electromagnetic clutch itself is complicated and that a structure of an apparatus is complicated because the apparatus requires a gear train as a gearshift mechanism shifting rotations from the motor beside the electromagnetic clutch.